When a signal on an optical disc is read by irradiating laser light or when a signal is recorded on an optical disc in an optical disc apparatus, in order to stably perform a recording and reproducing operation it is important to maintain the amount of laser light at a constant level. The laser light is emitted from a laser diode in an optical pickup, and the amount of laser light is controlled by an electric current supplied to the laser diode. However, the laser diode has a characteristic that an output thereof easily fluctuates by a temperature. Therefore, in order that the laser diode stably emits a fixed amount of light, a control current is required to be increased or decreased continuously, while ceaselessly monitoring the amount of emitted laser light. In the optical disc apparatus, conventionally a photo-diode called a front monitor is provided in the vicinity of the laser diode inside the optical pickup, and APC (Automatic Power Control) is performed based on light detected by the monitor so that the power of laser light becomes a desired value.
FIG. 1 shows a circuit configuration diagram indicating a conventional APC operation. Laser light emitted form a laser diode 1 enters a beam splitter 3 after passing through a grating 2 to be polarized. The beam splitter 3 reflects the incident laser light toward an objective lens 4 and transmits a part of the light toward a front monitor 6. The reflected laser light is irradiated on a disc 5 in a condensed state through the objective lens 4. The irradiated laser light is reflected by the disc 5, and the reflected light is detected by a detector 7 after passing through the beam splitter 3. Through an RF amplifier, a reproduction signal processing circuit, various servo signal processing circuits such as for focusing and tracking signals, and the like (not shown in the drawing) are connected to the detector 7. Also, the signal detected by the front monitor 6 is input into an APC circuit 8 consisting of sample-hold circuits S/H 10, S/H 11 and differential amplifiers 12, 13, and an output from the APC circuit 8 is input into a laser drive circuit LDD 9 to control the laser light output. Then, the APC operation is further explained separately for reproduction and for recording.
When reproduction is performed, the laser diode 1 emits light with a constant reproduction power. Reproduction signal processing is performed on a reproduction signal detected by the detector 7 to reproduce information and servo signals for focusing, tracking and the like are generated. Further, the signal detected by the front monitor 6 is input into the sample-hold circuits S/H 10 and 11. At this time, the sample-hold circuit S/H 10 is made to be in a state of sampling continuously. On the other hand, the sample-hold circuit S/H 11 is in a state of OFF and a sample-hold operation is not performed. An output of the sample-hold circuit S/H 10 is input into an inverting terminal of the differential amplifier 12 to be compared with a target value 14 of reproduction power which is a fixed value. The differential amplifier circuit 12 outputs a differential signal of reproduction power to the laser drive circuit LDD 9, so that the laser output light is controlled. Thus, in the APC operation when reproduction is performed, an operation of maintaining the reproduction power at a constant level is performed to stabilize a reproduction signal quality and various servo signals (for focusing, tracking and the like).
On the other hand, at the time of recording, the laser diode 1 alternately outputs the recording power that is larger than reproduction power and the reproduction power in accordance with an EFM signal output from an encoder as shown in FIG. 2A. The sample-hold circuit S/H 10 performs sampling of a front monitor detection signal at the timing of the reproduction power and holds at the time of the recording power as shown in FIG. 2B. The output of the sample-hold circuit S/H 10 sampled at the timing of the reproduction power is input into the laser drive circuit LDD 9 after being compared with a fixed target value 14 of the reproduction power in the differential amplifier 12, and the operation of maintaining the reproduction power at a constant level is performed to stabilize the reproduction signal quality and the various servo signals (for focusing, tracking and the like). On the contrary, the sample-hold circuit S/H 11 performs sampling of the front monitor detection signal at the timing of the recording power as shown in FIG. 2C and holds at the time of the reproduction power. The output of the sample-hold circuit S/H 11 sampled at the timing of the recording power is input into the laser drive circuit LDD 9 after being compared with a fixed target value 15 of the recording power in the differential amplifier 13, and an operation of maintaining the recording power at a constant level is performed to stabilize a recording signal quality.
[Patent reference 1] Japanese Published Patent Application No. 2003-141767
In the above described APC circuit, a fluctuation of laser light is detected based on the laser light emitted from the laser diode, and a control is performed to maintain the laser power at a constant level. However, when recording and reproduction are actually performed, there is a case in which an amount of light reflected from a disc varies between an inner circumference side and an outer circumference side due to a warp of the disc. In addition, there is a case in which a reflectance varies on the same disc depending on a position due to uneven coating of a recording material when manufacturing the disc. Further, the reflectance also locally changes due to a defect such as a finger print or a scratch on the disc. Due to the above, even if the laser power is maintained at a constant level by the above described APC circuit, the same reproduction signal level and servo signal level cannot be obtained continuously because of the fluctuation of light amount detected by the detector.
Furthermore, the APC operation on recording carries out the control while performing the sample-hold of two kinds of power, that is, of the recording power for stabilizing a recording quality and the reproduction power for stabilizing various servo signals, as described above; however, since the APC of the reproduction power controls much minute power compared with the APC of the recording power, such influence as a pulse light emission waveform of the laser diode, a pulse response (settling) of the front monitor, an offset of the sample-hold circuit and the like cannot be neglected. FIG. 2D shows an output waveform of the front monitor at this time. The higher a recording speed is, the more those errors occur, and a shift of the bottom power occurs, which greatly affects the various servo controls. In the above, the recording in a write-once type disc is referred to as an example; however there exists a similar problem in a case of recording on a rewritable type disc such as a phase change disc, though the APC operation is performed by carrying out the sample-hold of erasing power instead of the reproduction power.